Control of spangle in hot dip galvanizing



May 30, 1967 M. MONACO CONTROL OF SPANGLE IN HOT DIP GALVANIZING Filed Feb. 13, 1963 villi!!!III!!!Iflllllllfllll/If'fll/ffll w INVENTOR jfb c/zcze 270mm; BY

United States Patent 3,322,560 CONTROL OF SPANGLE IN HOT DIP GALVANIZING Michael Monaco, Bolton, 111., assignor to Inland Steel Company, Chicago, 11]., a corporation of Delaware Filed Feb. 13, 1963,-Ser. No. 258,337 9 Claims. (Cl. 11751) This invention relates to improvements in hot dip galvanizing of ferrous metal base material and more particularly to a method of reducing or minimizing the formation of surface spangle.

The familiar flowery spangle usually associated with galvanized steel is caused by crystals of zinc growing until they meet crystals growing from other directions. It is generally believed that such crystal growth is nucleated by uneven or localized cooling or by small particles of impurities in the molten zinc coating. The spangled surface of ordinary galvanized sheet material is uneven and irregular with pronouced depressions at the spangle boundaries.

For certain applications of galvanized steel, e.g. in the automotive industry, where the surface must be painted, the customary rough spangled surface is highly objectionable since it is difficult to conceal or cover the spangled appearance by ordinary painting methods. Consequently, there is at present an appreciable demand for galvanized naterial having a smooth substantially spangle-free surace.

Various methods have been proposed, and in some cases used, to eliminate or reduce spangle, but no consistently satisfactory, convenient, and economical technique has heretofore been developed. For example, cold rolling of spangled galvanized steel has sometimes been used to provide a smoother and less objectionable surface appearance. However, this process is expensive and also has an adverse effect on the ductility and formability of the product. Controlled heat treatment of galvanized material after it leaves the zinc pot, as described, for example, in US. Patent No. 3,056,694, produces a dull spangle-free product as a result of extensive alloying of the zinc coating with the ferrous base. However, this product likewise has certain limitations and is not suitable for many deep drawing applications.

Another method which has been used to eliminate spangle is the so-called steam blow in which wet steam is sprayed onto the galvanized strip as it leaves the zinc pot for its rapid cooling effect. It appears that the water particles in the wet steam cause nucleation and freezing of the molten zinc so rapidly that the crystals or spangles do not have a chance to grow. The result is that the spangles are extremely small so that the appearance to the naked eye is that of a dull spangle-free sheet. Though the steam blow principle is theoretically sound, in practice the technique requires a high degree of control and does not consistently produce a satisfactory product.

Accordingly, a primary object of the present invention is to provide a novel and improved method for obtaining a substantially spangle-free galvanized product.

Another object of the invention is to provide novel and improved means for eliminating or minimizing spangle in a galvanized product without adversely affecting the physical properties of the product.

A further object of the invention is to provide a novel and improved method of producing substantially spanglefree galvanized material which is convenient and economical to use and which is readily adaptable to continuous operation with strip material of varying width and thickness.

An additional object of the invention is to provide a novel and improved hot dip galvanizing process which is readily adaptable for the production of spangled or sub stantially spangle-free product, as desired.

Other objects and advantages of the invention will become evident from the subsequent detailed description taken in conjunction with the accompanying drawing which is a schematic view of an apparatus for practicing the invention.

As will hereinafter appear, the invention has particular significance with regard to a continuous strip hot dip galvanizing operation of the type utilizing an in-line heat treating step. For example, in the Sendzimir process the cold reduced steel strip is passed through an elongated furnace and thence downwardly through a snout or hood 'below the surface of the molten zinc bath. In the first portion of the furnace an oxidizing atmosphere is maintained to burn off oil or grease on the surface of the strip and also form a thin oxide coating. In a subsequent portion of the furnace a reducing atmosphere is maintained in order to reduce the oxide film. Following reduction of the oxide film the strip is cooled to a temperature somewhat above the pot temperature and enters the molten zinc bath through the snout or hood while protected by the reducing atmosphere. In another type of operation utilizing in-line heat treatment the strip is alkaline cleaned, rinsed and dried, annealed in a heating furnace containing a non-oxidizing atmosphere, cooled to slightly above pot temperature, and then introduced into the zinc bath through a snout or hood while still protected by the furnace atmosphere. In addition to the foregoing galvanizing methods, the principles of the invention are also utilizable in hot dip sheet galvanizing.

The customary zinc baths used in hot dip galvanizing contain significant quantities of lead as an impurity. It has long been known that the lead content of the zinc bath has an important influence on the size of spangle. For exemple, for the production of the usual spangled galvanized product it is customary in continuous strip hot dip galvanizing to maintain the lead content of the zinc bath within the range of from about 0.12 wt. percent to about 0.50 wt. percent in order to insure the desired large spangle while also obtaining other desired properties in the coated product. However, if it is desired to produce a substantially spangle-free product, it has been found that the lead content of the bath must be reduced to very low levels in order to produce the desired effect. In a given galvanizing line the production schedule is subject to frequent changes, and if the pot composition has been adjusted to obtain a very low lead content in order to make a substantially spangle-free product, it will require considerable time to adjust the lead content upwardly in the event that production of normal spangled material is necessary at a later time. Thus, control of lead content alone is not a sufficiently convenient, effective, and rapid technique for making a substantially spanglefree product under the usual plant conditions. 7 By the term substantially spangle-free it will be understood that the size of the spangle has been reduced sufficiently to give a uniform dull grey appearance to the naked eye. For example, in the conventional spangled product the spangle size is on the order of about 1 to 1 /2 inches in diameter, whereas in a substantially spangle-free product, as obtained by the process of the present invention, the spangle size is less than about Ms inch in diameter.

In accordance with the present invention, it has been discovered that even though .a substantially spangle-free product cannot be obtained in a consistent and convenient manner solely by controlling the lead content of the zinc bath, nevertheless, the problem is readily solved by the combined steps of 1) maintaining the lead content of the zinc bath below a critical maximum level and (2) maintaining the temperature of the ferrous base material below a critical maximum as the material enters the zinc bath. Broadly speaking, the desired substantially spanglefree product is obtained when the lead content of the zinc bath is restricted to about 0.10% max. and the strip or other ferrous base material has an inlet temperature substantially less than the temperature of the zinc bath.

In general, the molten zinc bath will have a tempera ture in the range of from about 850 F. to about 870 F. and usually from about 860 F. to about 865 F. Accordingly, to obtain a substantially spangle-free product in accordance with the present invention the inlet strip temperature in a continuous operation should be not greater than about 750 F. and preferably not greater than about 700 F., with 600 F. to 650 F. as a good operating range. It is well known that in a high speed continuous strip line it is difficult to obtain an accurate measurement of the strip temperature. However, for control purposes it has been found that good results are obtained by measuring the strip temperature with a properly calibrated radiation detecting instrument, such as a radiation pyrometer, at the turndown roll, i.e., the roll at the exit end of the furnace from which the strip passes downwardly through the snout into the zinc pot. The strip temperatures referred to herein are those obtained by an instrument of the aforementioned type.

In the conventional operation of a continuous strip galvanizing process the entry temperature of the strip is maintained somewhat above the zinc bath temperature in order to maintain the desired bath temperature with little or no heat input by the heating elements at the pot. Accordingly, in the present invention in which the entry temperature of the strip is substantially below the bath temperature it will usually be necessary to heat the pot in order to counteract the cooling effect of the strip. Although for purposes of reliably producing a substantially spangle-free product the inlet strip may have any temperature below the preferred maximum of 700 F., as a practical matter the inlet temperature should not be below about 450 F. in order to avoid an excessive cooling effect on the zinc bath with consequent excessive required heat input at the pot.

The above-described requirements for producing a substantially spangle-free product are most effective for light gauge steel or ferrous metal strip, e.g. from about 20 gauge to about 30 gauge. In the case of heavier gauge strip, e.g. up to about 14 gauge, best results are obtained if the coated strip emerging from the bath is also cooled by supplementary cooling means, such as air sprays, fine water sprays, or wet steam. However, even in the ease of the heavier gauge strip, the aforementioned dual control over lead content of the bath and strip entry temperature (without supplemental cooling) produces a very beneficial result in that the relief effect, i.e. the depth of the 'depressed spangle boundaries, is greatly diminished even though the reduction in spangle size is not as great. As a consequence, the spangle appearance is much less evident to the naked eye and for many end uses this product will be entirely acceptable. Moreover, even when supplemental cooling by air, water, or wet steam is considered necessary to obtain the maximum reduction in spangle size, the previously described practice of restricting the lead content of the bath and the entry temperature of the strip makes it possible to utilize a much lower water or steam pressure in the supplemental cooling step, thereby avoiding the danger of pitting or otherwise disfiguring the zinc coated surface of the strip. In some cases *where an exceptionally smooth spangle-free product is desired, it may be necessary to resort to temper rolling of the galvanized product so as to eliminate even the slightly rough surface of the substantially spangle-free product. However, the aforementioned practice in accordance with the present invention provides such a marked reduction in spangle size and relief that only a moderate extent of temper rolling is needed, thereby minimizing the adverse effect of temper rolling on the ductility and drawing qualities of the finished product.

In a continuous strip galvanizing operation it is highly desirable to be able to produce either the conventional spangled product or a substantially spangle-free product, as may be required, without the necessity of making major time-consuming and inconvenient changes in bath composition or operating procedure. It is a particular advantage of the present invention that this desired flexibility is provided in a remarkably effective and simple manner. Thus, by maintaining the lead content of the zinc bath at a suitable low level below the critical maximum, it becomes possible to otbain either type of product merely by adjusting the inlet or turndown roll temperature of the strip which can easily be accomplished by regulation of the cooling rate in the cooling portion of the furnace. For example, when the lead content of the bath is maintained at from about 0.05 wt. percent to about 0.10 wt. percent, a strip temperature at the turndown roll of about 600 to 650 F. will produce a substantially spangle-free product. However, merely by raising the strip temperature at the turndown roll to about 800 to 850 F. the normal large size spangle returns promptly. Thus, the present invention not only affords a highly effective solution to the problem of spangle elimination but also accomplishes this result in a manner which permits either type of product to be made at will.

Referring now to the drawing, a typical apparatus for practicing the invention is illustrated. Low or medium carbon cold reduced steel strip 10 is fed from a supply roll 11 and is passed through an oxidizing furnace 12 provided with gas burners 13 at both sides of the strip. The strip 10 is heated in the furnace 12 under oxidizing conditions so as to burn off oil or grease and form a thin oxide coating or film. The strip 10 then enters an elongated furnace having a reducing section 14 and heating means 15 such as burners or the like. The furnace is supplied with a reducing atmosphere (by means not shown) and sealing rollers 16 or the like are provided at the furnace inlet. In the furnace section 14 the oxide coating on the strip 10 is reduced and the strip then passes through a cooling section 17 having cooling tubes 18 or the like. The cooled strip next enters a snout 19 and passes downwardly over a turndown roll 20 and thence beneath the surface of the molten zinc bath 21 in a pot 22 having heating means (not shown). A temperature measuring instrument 23 (such as a Minneapolis-Honeywell RL Electronik Radiamatic pyrometer) is mounted above the turndown roll 20 for measuring the strip temperature at this point. The strip 10 passes beneath a sinker roll 24 immersed in the zinc bath 21 and then passes upwardly between a pair of coating rolls 25 at the bath surface. As the zinc coated strip leaves the pot, the coating may be cooled rapidly by banks 26 of cooling sprays of air, water, wet steam, or combinations thereof and the strip then passes over a guide roll 27. As previously described, the cooling sprays 26 may or may not be used dependent upon the gauge of the strip and other factors. In accordance with the present invention the lead content of the zinc bath 21 is maintained within the previously disclosed limits. Moreover, the strip temperature at the turndown roll 20 as measured by the pyrometer 23 is regulated by the cooling tubes 18 so as to obtain either a normal spangled product (e.g. at a temperature of 800850 F.) or a substantially spangle-free product (eg at a temperature of 600650 F.)

Although the herein described practice for producing small spangle or substantially spangle-free galvanized strip was discovered empirically, the following theoretical explanation of the phenomenon is presented to illustrate the heretofore unappreeiated coaction between lead content of the bath and strip entry temperature. In general, rapid cooling of the zinc coating is essential for obtaining small spangle size. The required cooling may be accomplished in a number of ways, but in accordance with the present invention it has been found that the most effective technique is cooling of the base strip before hot dip coating so that the resultant Zinc coating is cooled from within. Since spangle is in reality the grain of the zinc coating, cooling from within is an ideal approach. Consulting the zinc-lead equilibrium diagram shows that rapid cooling down through the portion of the diagram in which liquid phases are present will tend to inhibit the rejection of lead from the liquid metal being cooled. It is believed that the rejection or precipitation of lead is responsible for providing the nuclei for large spangle boundaries and deep relief or boundary roughness. Accordingly, by maintaining a low lead content in the zinc bath and rapidly cooling the applied coating [from within, it will be seen that there is only a slight amount of lead available to form large spangle and the small amount present is effectively prohibited from precipating out by the rapid cooling action.

I claim:

1. In a hot dip galvanizing process wherein a heated ferrous metal base is passed through a molten zinc bath containing a small amount of lead and having a temper ature of from about 850 F. to about 870 F., the improvement which comprises maintaining the lead content of the bath Within the range of from about .05 wt. percent to about .10 wt. percent, said bath being free of other elements in amounts having any significant effect on spangle, and controlling the temperature of said base so that it enters the bath at a temperature not greater than about 750 F., whereby to reduce spangle formation in the galvanized product and whereby a product having increased spangle can be obtained when desired by raising the entry temperature of the base without altering the bath composition.

2. The process of claim 1 further characterized in that the temperature of said base as it enters the bath is not greater than about 700 F.

3. In a continuous galvanizing process wherein a ferrous metal strip is passed through an in-line heating step and is then passed through a molten zinc bath containing a small amount of lead and having a temperature of from about 850 F. to about 870 F., the improvement which comprises regulating the composition of the zinc bath so that the lead content thereof is within the range of from about .05 wt. percent to about .10 wt. percent, said bath being free of other elements in amounts having any significant effect on spangle, and maintaining the strip just prior to immersion in the zinc bath at an entry temperature not greater than about 750 F., whereby to reduce spangle formation in the galvanized product and whereby a product having increased spangle can be obtained when desired by raising the entry temperature of the strip without altering the bath composition.

4. The process of claim 3 further characterized in that the entry temperature of said strip is not more than about 700 F.

5. The process of claim 3 further characterized in that said strip is passed suc-cessively through a heating zone and a cooling zone and is then passed downwardly over a turndown roll into said bath, said entry temperature being the temperature of the strip as measured at said turndown roll.

'6. The process of claim 5 further characterized in that said strip entry temperature is measured at said turndown roll by a radiation pyrometer.

7. The process of claim 3 further characterized in that said strip entry temperature is not less than about 450 F.

8. The process of claim 3 further characterized in that the zinc coated strip emerging from said bath is cooled by means of a fluid cooling medium.

9. The process of claim 8 further characterized in that said cooling medium comprises wet steam.

References Cited UNITED STATES PATENTS 2,094,583 10/1937 Cook 117-51 2,245,225 6/ 1941 Renkin 117-51 2,703,766 3/1955 Ellis 117-114 2,824,020 2/1958 Cook et al 117-114 3,148,080 9/1964 Mayhew 117-114 X 3,148,081 9/1964 Ross l17-114X ALFRED L. LEAVITT, Primary Examiner. RALPH S. KENDALL, Examiner. 

1. IN A HOT DIP GALVANIZING PROCESS WHREIN A HEATED FERROUS METAL BASE IS PASSED THROUGH A MOLTEN ZINC BATH CONTAINING A SMALL AMOUNT OF LEAD AND HAVING A TEMPERATURE OF FROM ABOUT 850*F. TO ABOUT 870*F., THE IMPROVEMENT WHICH COMPRISES MAINTAINING THE LEAD CONTENT OF THE BATH WITHIN THE RANGE OF FROM ABOUT .05 WT. PERCENT TO ABOUT .10 WT. PERCENT, SAID BATH BEING FREE OF OTHER ELEMENTS IN AMOUNTS HAVING ANY SIGNIFICANT EFFECT ON SPANGLE, AND CONTROLLING THE TEMPERATURE OF SAID BASE SO THAT IT ENTERS THE BATH AT A TEMPERATURE OF NOT GREATER THAN ABOUT 750*F., WHEREBY TO REDUCE SPANGLE FORMATION IN THE GALVANIZED PRODUCT AND WHEREBY A PRODUCT HAVING INCREASED SPANGLE CNA BE OBTAINED WHEN DESIRED BY RAISING THE ENTRY TEMPERATURE OF THE BASE WITHOUT ALTERING THE BATH COMPOSITION. 